The United States government took a historic step back from its long-running drug war on Thursday, when Attorney General Eric Holder informed the governors of Washington and Colorado that the Department of Justice would allow the states to create a regime that would regulate and implement the ballot initiatives that legalized the use of marijuana for adults.

A Justice Department official said that Holder told the governors in a joint phone call early Thursday afternoon that the department would take a "trust but verify approach" to the state laws. DOJ is reserving its right to file a preemption lawsuit at a later date, since the states' regulation of marijuana is illegal under the Controlled Substances Act.

Deputy Attorney General James Cole also issued a three-and-a-half page memo to U.S. attorneys across the country. "The Department's guidance in this memorandum rests on its expectation that states and local governments that have enacted laws authorizing marijuana-related conduct will implement strong and effective regulatory and enforcement systems that will address the threat those state laws could pose to public safety, public health and other law enforcement interests," it reads. "A system adequate to that task must not only contain robust controls and procedures on paper; it must also be effective in practice."

The memo also outlines eight priorities for federal prosecutors enforcing marijuana laws. According to the guidance, DOJ will still prosecute individuals or entities to prevent:

the distribution of marijuana to minors; revenue from the sale of marijuana from going to criminal enterprises, gangs and cartels; the diversion of marijuana from states where it is legal under state law in some form to other states;

state-authorized marijuana activity from being used as a cover or pretext for the trafficking of other illegal drugs or other illegal activity; violence and the use of firearms in the cultivation and distribution of marijuana drugged driving and the exacerbation of other adverse public health consequences associated with marijuana use;

growing of marijuana on public lands and the attendant public safety and environmental dangers posed by marijuana production on public lands; preventing marijuana possession or use on federal property.

The eight high-priority areas leave prosecutors bent on targeting marijuana businesses with a fair amount of leeway, especially the exception for "adverse public health consequences." And prosecutors have shown a willingness to aggressively interpret DOJ guidance in the past, as the many medical marijuana dispensary owners now behind bars can attest.

U.S. Attorneys will individually be responsible for interpreting the guidelines and how they apply to a case they intend to prosecute. A Justice Department official said, for example, that a U.S Attorney could go after marijuana distributors who used cartoon characters in their marketing because that could be interpreted as attempting to distribute marijuana to minors.

But the official stressed that the guidance was not optional, and that prosecutors would no longer be allowed to use the sheer volume of sales or the for-profit status of an operation as triggers for prosecution, though these factors could still affect their prosecutorial decisions.

The Obama administration has struggled with the legalization of medical marijuana in several states. Justice Department Officials had instructed federal prosecutors across the country not to focus federal resources on individuals who were complying with state laws regarding the use of medical marijuana. But the U.S. attorneys in several states that had legalized medical marijuana rebelled, and what was known as the Ogden memo faced stiff resistance from career prosecutors.

"That's just not what they do,” one former Justice official told HuffPost. “They prosecute people."

As a result of the internal pushback at DOJ, a new memo was issued by Deputy Attorney General James Cole in 2011 that gave U.S. attorneys more cover to go after medical marijuana distributors. Federal prosecutors began threatening local government officials with prosecution if they went forward with legislation regulating medical cannabis.

After recreational marijuana initiatives passed in Washington and Colorado in November, President Barack Obama said the federal government had “bigger fish to fry” and would not make going after marijuana users a priority.

Holder said back in December that the federal response to the passage of the state ballot measures would be coming “relatively soon.”

Washington Attorney General Bob Ferguson told HuffPost his office was preparing for the “worst-case scenario” of a federal lawsuit against the law.